The presence or non-presence of clouds in a region is a consequence of complex interactions among many atmospheric, oceanic and terrestrial processes. It is a visualization of changes in a number of environmental factors, such as radiation, heat, temperature, moisture, precipitation, etc. All these factors are vital for the functioning of terrestrial ecosystems, and influence the exchange of energy and mass of terrestrial ecosystems with the overlying atmosphere. In particular, it has been observed that for many forest ecosystems, the maximal net uptake of carbon dioxide (CO2) often occurs on cloudy rather than on sunny day. In a previous study, we showed that a boreal deciduous forest and a temperate deciduous forest had maximal carbon uptake under sky conditions with a solar radiation level equivalent to about 70-80% of the corresponding clear-sky solar irradiance. We also demonstrated that the two forests could tolerate a clouds-induced sunlight reduction of as much as 50% without lowering the capacity in carbon sequestration as compared with sunny days. However, uncertainties remain in regard to the mechanism of this phenomenon. Since variations in net carbon uptake can be achieved through changes in photosynthesis, or respiration or both, an interesting question is: does moderate cloudiness increase net carbon uptake by enhancing canopy photosynthesis or by reducing ecosystem respiration or both? In this study, we have investigated the responses of a variety of terrestrial ecosystems to cloudiness by analyzing tower flux measurements collected through the Fluxnet project. We found that moderate cloudiness tends to enhance canopy photosynthesis. For the influences of clouds on ecosystem respiration, the responses are more complex. Several ecosystems show reduction in ecosystem respiration in the presence of clouds, but not for others. Possible explanations for this difference include temperature-moisture interactions on soil respiration.